A floating gate-type flash memory device has heretofore been used as a nonvolatile memory allowing electrical writing, reading and erasing. A flash memory of this type has a three-layered structure of polysilicon-insulator-polysilicon wherein a floating gate and a control gate are formed of polysilicon films, respectively and an interlayer insulating film, for example, made of silicon oxide or silicon nitride is sandwiched by the polysilicon films.
FIG. 5 shows the structure of such a flash memory. In FIG. 5, a semiconductor substrate 1 is covered with a silicon oxide film 4 functioning as a gate insulating film on which sequentially a floating gate 5 formed of a polysilicon film, an interlayer insulating film 6 formed of a silicon oxide film and a control gate 7 formed of a polysilicon film are stacked. Further, n-type impurity diffusion formed regions are in the p-type silicon substrate 1 on opposite sides of the gate electrode to form a source region 2 and a drain region 3. On the substrate surface, a LOCOS film 8 for device isolation, and a n.sup.- -type lightly doped region 9 underlies the source region 2 for enhancing the dielectric strength of the memory device is formed.
When the flash memory thus arranged is to be written with data, voltage is applied to a region interposed between the drain region 3 and the control gate 7 to inject electrons into the floating gate 5, and the amount of electrons to be injected is controlled to vary depending on which data component "1" or "0" is to be stored. The stored data can be erased by applying voltage inversely, or across the control gate and the source region 2 to drain the injected hot electrons. Then, the flash memory becomes ready to be rewritten.
Conventionally, a rewritable and erasable flash memory of this type is frequently made to coexist in a semiconductor device with a read only memory (ROM) which is not adapted for erasing of data once stored therein. In order to simplify the production process for such a semiconductor device, however, the semiconductor device is produced with all the memory therein composed of a rewritable and erasable flash memory device even if a ROM is needed therein, and then some of the flash memory devices are electrically written for use as a ROM.
As described the above, the ROM is produced by the same process as the flash memory and have the same structure as the flash memory. For this reason, the ROM cannot be used as a ROM without writing a state of "1" or "0". Then, the ROM without reading "0" or "1", cannot be used as a memory. Accordingly, there is such a problem that the memory for use as a ROM needs to be written upon use.